1. Field of the Invention
The present invention relates to an apparatus where the wet of several different gas flows is carried out in at least three scrubbing towers, and the droplet separation of the scrubbed gases takes place in a uniform cyclon-type droplet separator, so that the obtained product is a pure, dropless gas.
2. Description of Related Art
Presently the tightened legislation in environmental protection sets higher and higher demands on the cleaning of exhaust gases. Above all this concerns industry and may become a threshold issue for many company, as for the continuation of production. This also means that more and more companies will invest in gas scrubbing, both in development and production as well as in new purchases.
In the majority of conventional practices, attention is paid to the scrubbing phase only, with the confidence that some separate, ready-made filter serves well enough as the droplet separator. This means that the scrubber units easily become bulky groups of miscellaneous constructions, if there are more gas lines than one. It is, however, pointed out that easy maintenance and effective droplet separation are in most cases almost as important as the scrubber unit itself.
An ideal solution for such cases is a combined scrubber and droplet separator unit, where the exhaust gases from different stages can be conducted separately and discharged directly into open air.
In principle, the scrubbing of gases can thus be divided into two stages, i.e. the scrubbing proper and the successive droplet separation. The scrubbing methods and requirements largerly define what kind of scrubber is best. The same applies to droplet separators.
A very popular principle is the venturi principle. In practice this includes two different types. One type uses a water spray to give strength and energy to the scrubbing, and at the same time the spray serves as a kind of blower, a suction fan. In the other type, the gas itself renders strength to the process, in which case the gas must be pressurized.
Both of the above mentioned scrubber types are described to a great detail in literature, for example in the following references: (1) H. Brauer, Y. B. G. Varma: Air Pollution Control Equipment, Springer-Verlag 1981, or (2) W. Strauss: Industrial Gas Cleaning, Pergamon Press 1975.
One problem when using venturi scrubbers, as well as with other scrubber types, is changing the capacity. The patent specification FR 2,452,311 introduces a system where a collar restricting the gas flow is formed in the venturi shaft by means of an annular water layer. By adjusting the thickness of the water layer, any changes in the capacity can be easily responded to.
Another, perhaps even more problematic situation is created when the capacity rises to a higher order. In that case the range of the water spray in relation to the gas flow is not sufficient, no matter which of the two venturi systems is employed. In order to solve this problem, for instance the reference (1), on pages 109 and 110, introduces applications with several venturis instead of only one.
Centrifugal force can also be made use of in gas scrubbers. A wet cyclone based on this idea is described for instance in the U.S. Pat. No. 2,696,275 and on the page 374 of reference (2), where a centrifugal wet scrubber and a turbulence phenomenon created by means of guiding vanes is introduced.
From the U.S. Pat. No. 3,456,928, there is known a scrubber based on a kind of a blower effect, where the gas flow is allowed to blow towards the surface of the water collected on the bottom of the scrubber.
Upwards rising gas to be cleaned can be put to contact with downwardly directed water sprays by using a cylindrical scrubbing vessel, as is described in the U.S. Pat. No. 3,773,472. The direction of the two flows can also be opposite. A similar type of scrubbing method, where the scrubbing cylinder is filled with filler particles, is described for instance on pages 122-123 and 129 of reference (1).
Different types of droplet separators are described for instance on pages 219-239 of reference (1). The main droplet separator types are based on the zig-zag principle, centrifugal principle or conventional filter principle.
According to the present invention, the gas or gases to be scrubbed are conducted into at least three vertical scrubbing tubes, and thereafter into a uniform droplet separator formed of several nested cylinders, where the gases are forced in a rotary motion and the pure and droplet-free gas is drained through a drain tube located in the middle of the scrubbing tubes. The essential novel features of the invention are apparent from the appended patent claims.
An advantage of several scrubbing tubes is that even with large gas volumes, the scrubbing process still remains well under control. In droplet separation, the uniform treatment of even large amounts of gases has not caused problems--on the contrary, from the point of view of building technique, it is simpler to construct a compact apparatus where all of the gas, even a large amount, is conducted to. Nowadays, owing to the said strict air-protective legislation, the tendency is to clean the gases of more and more components, and consequently the scrubber of the present invention with several scrubbing tubes is well suited to this purpose. At the droplet separation stage, solid particles have already been removed, and therefore it is even more recommendable to treat the gases flexibly together. Moreover, the apparatus is particularly suitable for treating emissions, because after cleaning, all of the gases can be discharged into open air.
Gases are conducted into the scrubbing tubes either separately, or after first combining them in the preliminary chamber and then distributing them into the tubes. The scrubbing tubes can be mutually similar or different, both in operation and size, depending on the scrubbing requirements--advantageously, however, they are similar. Because the tubes can differ from each other, it is possible to use the apparatus for treating emissions from different types of sources, and each emission type can be treated in a unit particularly designed for it. Thus the scrubbing tubes can be for instance a venturi scrubber, a centrifugal wet scrubber, a spray-type scrubber or a filler scrubber. The essential point is that the scrubbing tubes are located symmetrically with respect to each other. After the scrubbing process, the scrubbed gases are allowed to collide in the water gathered in the largest cylindrical vessel of the scrubber of the invention, which collision leads to the second stage of the scrubbing process.
The gases scrubbed in the scrubbing tubes are tangentially conducted, each along its specific conduit, to the next cylinder. Only in this cylinder, the gases are combined. The tangential path is arranged by means of auxiliary walls in the chamber and by possible blade extensions. This second largest cylinder begins, in the vertical direction, from the ceiling of the scrubbing chamber, and ends--as open--underneath the water level. Thus conditions for a simple and effective droplet separation are created; for the tangential feeding of gases secures the operation of centrifugal forces, so that the rotary gas flow is burst into the next inner cylinder through the annular opening located in between this inner cylinder, its bottom part and the water surface. From this inner cylinder, the gas is discharged, through the smallest cylinder, and according to a normal cyclone effect, upwards droplet-free.
The scrubbed, droplet-free yet saturated gas can be conducted directly to the open air or, if conditions particularly require, through a heat exchanger formed by the hot gases entering the scrubber for cleaning, in which case the temperature of the gas to be drained rises to a level where water is not condensed on the possibly cold walls of the drain pipe. This fact is observed particularly when scrubbing in unheated facilities.